1. The Field of the Invention
The invention relates to a laser radar apparatus and a method for measuring both the direction of an object and the distance to an object from the apparatus using a laser beam.
2. Description of the Prior Art
As laser radar apparatus capable of measuring both the direction of an object and the distance to an object from the apparatus, there is a known apparatus such as disclosed by Hoashi et al. in Japanese Patent No. 2789741.
The laser radar apparatus of Hoashi et al. includes a laser beam generating means for emitting a laser beam or a laser pulse serving as an outgoing light into a measurement range, the outgoing light having an optical axis thereof, a photo detecting means for detecting the reflected laser beam or the reflected laser pulse that arrives after the outgoing light is reflected by an object located in the measurement range and serve as an incoming light, an optical isolator that allows the outgoing light to transmit therethrough and forbids the incoming light to pass, and an electric control unit (ECU). The laser beam generating means is, for example, a laser diode that produces laser emission including a laser beam and a laser pulse. The photo detecting means is, for example, a photo diode that converts an incident laser beam or an incident laser pulse to an electric current that is a function of intensity of the incident laser beam or the incident laser pulse. The optical isolator reflects the incoming light, and the incoming light reflected by the optical isolator will be directed to the photo detecting means. In order to realize these functions, it is preferable that the optical isolator is located on the optical axis of the outgoing light. The electric control unit (ECU) calculates a distance from the apparatus to the object, if it exists, based on the difference of the phases of the outgoing and incoming laser beams, or the time of flight between the emission and reception of the laser pulse utilizing the speed of light. Further, the laser radar apparatus of Hoashi et al. includes a concave mirror that deflects the outgoing light toward the measurement range and the incoming light reflected back by the object toward the photo detecting means. Further the concave mirror is arranged to rotate up to 360 degrees so that an angular scanning range in the horizontal direction can be realized to be of up to 360 degrees. It should be noted that in the optical laser apparatus of Hoashi et al., a projection optical system includes the laser beam generating means, the optical isolator, and the concave mirror, and a photo detecting system includes the concave mirror and the optical isolator. The projection optical system and the photo detecting means are arranged coaxially in part. In more detail, the axes of the outgoing light and the incoming light between the optical isolator and the object are identical.
As described above, in the laser radar apparatus of Hoashi et al. or a laser radar apparatus of similar type, the axes of the outgoing light and the incoming light are identical, and the optical isolator is arranged to be located on the common axis to the outgoing light and the incoming light. The outgoing light emitted by the laser beam generating means pass through the optical isolator although the incoming light reflected back by the object is reflected from the optical isolator. In general, attenuation of the laser beam or the laser pulse may be caused during both the transmission and the reflection of the laser beam or the laser pulse through and from the optical isolator, respectively. Hence, beam splitting efficiency is degraded during the transmission and the reflection of the laser light through and from the optical isolator. This leads to a special configuration of some elements of the optical laser radar apparatus, for example, a bigger mirror having a larger mirror plate to enlarge an effective photo-receiving area so as to improve the beam splitting efficiency. This conflicts with the tendency of reducing the size of the apparatus.